It is theft under common sense.

Only in the sense that you reading a book from the library also constitutes theft of knowledge. Does it?

If I stole someone's private research notes and republished them loosely in my own words, they'd correctly be pissed

This was unpublished research that was stolen, and constitutes plagiarism and academic fraud by even the strictest definition

They are not “private research notes” if they are generated by an LLM.

The prompts they put in are the equivalent

>This was unpublished research that was stolen, and constitutes plagiarism and academic fraud by even the strictest definition

It was not stolen, it was willingly given.

The terms of service does not dictate what constitutes plagiarism